Science has come up with some numbers to back the long-held belief that nutrients washing out to sea trigger crown-of-thorns starfish outbreaks on Australia's Great Barrier Reef.

Dr Glenn De'ath of the Australian Institute of Marine Science and colleagues announced their findings at a recent conference 'Catchment to reef: water quality issues in the Great Barrier Reef region' held in Townsville.

Crown-of-thorns starfish feed on living coral. Huge numbers can cause problems as these spiny starfish eat coral faster than the coral can grow and reproduce.

De'ath told ABC Science Online there was a lot of circumstantial evidence to support the role of nutrient run-off in starfish outbreaks.

Nutrients in agricultural fertilisers, from grazing and urban development run off into reef waters.

For a long time, scientists have thought that a higher level of nutrients at sea led to increased levels of phytoplankton for the starfish larvae feed on.

"So if there is going to be more food then there is going to be more larvae," he said. "But we haven't been able to quantify this before to see how big the effects would be."

To quantify the effect of nutrients, the researchers developed a computer model to mimic the pattern and frequency of starfish outbreaks up and down the reef.

The model integrated findings from a range of studies on nutrient levels and starfish survival rates and the relationship between them. It also included information on the southward movement of currents that transport larvae along the reef.

Larvae love plankton

De'ath said evidence showed that doubling the larvae food source meant 10 times as many starfish survived.

Given that the starfish have a seven year life cycle and they start reproducing after two or three years, the model showed that doubling the food source can lead to numbers increasing 1000-fold in just 12 years.

In some areas of the reef south of Cairns nutrient levels are twice as high as those north of Cooktown.

The researchers assumed that the levels in the far north, where conditions are more pristine than further south, were close to what they were before European settlement and agriculture 150 years ago. So, they ran the model starting at these northerly conditions.

The researchers started the model at minus 150 years, and ran it forward, slowly increasing the level of nutrients to double the amount.

They found the number of starfish outbreaks increased from one outbreak in 100 years to the frequency seen today of one in 15 years.

There are two other main theories about the cause of increasing starfish outbreaks.

The idea that they are a natural phenomenon is hard to prove, said De'ath. While starfish spines dating back thousands of years have been found in reef sediments, there is no way of telling how frequently outbreaks occurred at the time.

Another theory is that fishing has reduced predation on larva. But De'ath said this was unlikely to have brought about the magnitude of change in starfish survival rates required to cause outbreaks.

"We would have had to had a 90% decrease in the number of predators," he said.